C - Lublin

ORGANIC SOIL CONSTITUENTS
Acidic functional groups of soil organic matter (aliphatic and aromatic carboxyls,
sulfoxyls, phenolic, enolic etc.) have very different acidic strength, depending
not only on the kind of the group, but also on its locality. An increase of the pH
of the soil solution leads to the neutralization of these groups thus they become
negatively charged. Surface groups of stronger acidic character (similarly as
stronger acids) are neutralized at lower pH values. The weaker acidic is the group,
its neutralization requires higher pH value. Therefore the higher is the soil pH, the
larger surface charge occurs on organic matter surfaces. These acidic surface
groups, which are located closely to each other, create common electric field surrounding
these groups and within this field their protons become delocalized (proximity
effect). The delocalized protons behave as strong acids and so the neighboring
groups are strongly acidic and the soil organic matter has some negative charge
even at very low pH values. The negative charge at pH around 8.2 of fulvic acids
reaches a few hundreds centimoles per kg and for fulvic acids up to one thousand.
PLANT ROOTS
On surfaces of plant roots, variable charge of carboxylic groups dominate. The
charge of plant roots was found to be closely correlated to uronic acid, present as
polymerized galacturonic acid in pectic substances (Knight et al. 1961). The magnitude
of root charge (CEC) was used for an explanation how various plant species
survive environments with low level of cations availability (Gray et al. 1953), why
plants uptake different proportion of mono and divalent cations (Huffaker and Wallace
1958), how plants compete in nutrient deficient mixed populations (Woodward
et al. 1983), what are the mechanisms of aluminum toxicity (Keltjens 1995).
This may be connected also with the surface charge density. The higher the SCD,
the higher the surface potential and the higher relative adsorption of multivalent
cations. Significant positive correlation between CEC of cell walls and Al sorption
was found by Schmol and Horst (2001), who postulated that Al binding by pectin
matrix is an important step in the expression of Al toxicity. The roots charge is
higher for Al sensitive plants than for Al resistant plants.
REFERENCES
1. Chi Ma and Eggleton, R.A. Cation exchange capacity of kaolinite. Clays and Clay Minerals
47, 174–180.
2. Gray B, Drake M and Colby W G 1953 Potassium competition in grass-legume associations
as a function of root cation exchange capacity. Soil Sci. Soc. Am. Proc. 17,
235-239.
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